I'm a Leanpub cofounder and just wanted to mention here that self-publishing a book while you're writing it is not necessarily incompatible with eventually going with a conventional publisher (though if you have a publisher in mind, you might want to ask them first if this would be acceptable to them). Publishing your book while you write it can help with motivation and the feedback you get from early adopter readers can be really valuable and make the book better in the end.
I love Leanpub and buy a ton of books and contribute as much as I can there to works in development. The whole feedback process has 100% won me over! If I ever published a book (something I've fantasized about for years), it would be probably through Leanpub or self-published. I'd probably consider one of the smaller presses, but Leanpub would be my go-to. You guys do great work at Leanpub!
